Atmospheric pressure ionization sources are widely used to study ions of biological interest. One of the most widely used sources is electrospray ionization, in which the liquid is delivered to the tip of a needle that is held at high voltage relative to some counter-electrode. The strong field around the tip of the needle serves to atomize the liquid, to charge the droplets and to push ions formed in the spray towards the counter electrode. Many names are applied to this technology including nanospray, nanoelectrospray, picospray, and turboionspray as some non-limiting examples of this class of ionization source. When coupled to this type of atmospheric pressure ionization source, a mass spectrometer, specifically a vacuum chamber of a mass spectrometer, generally has only a single opening to atmospheric pressure. Additional openings are not very practical for two reasons: (i) the introduction of a higher flow of gas into the vacuum system requires higher vacuum pumping capability, and (ii) combining the ions flowing through two or more openings is difficult inside the vacuum chamber of the mass spectrometer. The single opening into the mass spectrometer makes the delivery of ions from two or more ionization sources problematic, although several methods have been proposed and utilized.
Since FAIMS operates at atmospheric pressure, the option of utilizing multiple inlets is more readily exploited. Some examples of apparatus and methods associated with delivering the ions from several ion sources have been described in previous patent applications, such as for example U.S. Provisional Application No. 60/505,868, the contents of which are incorporated herein by reference.
Previously, multiple ion inlet FAIMS systems have been designed for maximizing the efficiency of operation of the mass spectrometer. For example U.S. Pat. No. 6,753,522 issued Jun. 22, 2004 in the name of Guevremont et al., the contents of which is incorporated herein by reference, teaches providing the sample flow from two separate flow injection systems to two independent ionization sources, which are then multiplexed into FAIMS through two ports. The selection of an active port is achieved by mechanical or electronic selection, with the objective being to deliver ions from each source during a period of time in which the alternative or non-active source cannot deliver sample (i.e. the alternative source is between samples). In this way the mass spectrometer is utilized more efficiently, with less time spent waiting for the next useful sample to be delivered. In another example, one of the ion sources is used for the calibration of the mass scale of a high-resolution mass spectrometer. The reference compound is delivered occasionally via one of the ion sources to periodically refresh the mass calibration.
It would be advantageous to increase the sensitivity of the mass spectrometer measurement by delivering a higher flow of ions through FAIMS. For example, this may be accomplished using several ionization sources that are arranged in parallel, all of which are ionizing the same sample. Normally this has not been done because the effort that is required to operate two sources is significant. In addition, since the mass spectrometer has only one opening, providing ions from several sources is not very practical nor does it provide higher sensitivity. This is because, under normal circumstances, additional ion sources mean that each source must necessarily be located further from the single opening into the mass spectrometer. As a result, each source is coupled less effectively and the ion flux from a given source into the mass spectrometer is lower than it would be if the ion source were located in an optimal position adjacent to the ion inlet into the mass spectrometer.